If the work described in any building permit has not begun within 60 days from the date of issuance thereof or if the work described in any building permit has not been substantially completed within 18 months of the date of issuance thereof, the permit shall expire and be cancelled by the City Clerk and written notice thereof shall be given to the persons affected, together with notice that any work as described in the cancelled permit shall not proceed unless and until a new building permit has been obtained; provided, however, times for commencement or completion may be extended by the Planning Commission prior to the expiration of the time periods; and provided further, no building or other structure in the city shall be allowed to remain in an unfinished condition for a period of over six months nor shall any building or other structure in the city to be allowed to remain in an unsafe, damaged, or demolished condition for a period of more than 60 days
(Prior Code, § 9-311)